Remember the days when marketers wanted nothing to do with social media and its questionable content? Those days are long gone. Facebook is now poised to overtake Yahoo in U.S. display ad revenues this year, according to revised estimates from eMarketer.

To be precise, eMarketer estimates Facebook's share of U.S. online display ad revenues will grow to 17.7% in 2011 -- up from a 12.2% share last year, garnering an expected $2.19 billion in display this year.

Google, meanwhile, should remain the third-largest seller of display ads domestically, according to eMarketer.

As the overall display ad marketplace improves, however, U.S. display ad revenues at Google will top $1 billion for the first time in 2011, as the company's share of overall U.S. display revenues is expected to grow to 9.3%. That's up from an 8.6% share in 2010, when Google's revenues grew an estimated 140.5% to $855 million.

By 2012, Google will be essentially in a dead heat with Yahoo, while Facebook will represent about one in five display ad dollars.

"Google's display revenue gains will come from three main sources," explained David Hallerman, principal analyst at eMarketer. "Large advertisers that are already Google customers; the range of small or medium-sized businesses that have relied on search for years but are looking to expand their reach; and large-brand marketers looking to YouTube to widen their video advertising reach."

Display business at Yahoo, Microsoft and AOL will also grow this year, although each company's market share will decline as a result of Google's and Facebook's huge rise, Hallerman notes.

Yahoo's share of the U.S. display ad market is expected to decline to 13.1% in 2011 -- down from 14.4% in 2010, eMarketer estimates.

In search, Google is expected to extend its lead over Microsoft and Yahoo this year, seemingly unaffected by the rapid growth of Bing.

What's more, eMarketer estimates net U.S. search ad revenue at Google will grow 38.9% to $10.92 billion in 2011. That will help push Google's share of overall search revenues to 75.9% this year, up from a 73.6% share in 2010 and a 69.8% share in 2009, according to Hallerman.

Microsoft's share of overall U.S. search ad revenues is expected to grow to 8% this year, when the company will bring in an estimated $1.15 billion in net search ad revenues, up 38.9% from $828 million and a 6.9% share of the total market in 2010.

The estimates in this most recent forecast for Facebook ad revenues were calculated in January 2011. This revision has adjusted the company's market share figures downward to account for the dramatic growth forecast in eMarketer's June 2011 U.S. online advertising estimates, especially in the display category.